As a Bulgarian citizen working in Germany, this is only partially the case. Germany does require work permits for Bulgarian citizens EMPLOYED here (except for certain exempt categories, such as scientific staff) but this does not apply to self-employment. In fact, there are no nationality restrictions for self-employment within the EU, and such restrictions would be illegal. So Bulgarian/Romanian citizens do need a permit to be employed within the EU countries that are still paranoid about their employment market, but don't need one to work there if self-employed.